We start by associating character instances across center frames, which ensures that each instance in \(B^m_i\)  is matched to at most one instance in \(B^m_j\) . Inspired by SORT [1]}, we formulate this association as a linear assignment problem with assignment cost computed using the negative IoU of instance masks. We denote \(A^m_{(i, j)}\)  as the set of associated instance mask pairs between \(c^m_i\)  and \(c^m_j\) . The instance-level IoU (IIoU) is computed as follows:
\(IIoU(B^m_i, B^m_j) = \dfrac{ \sum _{ (b^m_{(i, x)}, b^m_{(j, y)}) \in A^m_{(i,j)} }{ |b^m_{(i, x)} \cap b^m_{(j, y)}| } }{ |b^m_i \cup b^m_j| }\) 
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